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Floats in a Network
FLOAT !!!
This is in continuation of a hub: Case Study: PERT/CPM – calculating floats. Some terms used in the said hub would be repeated for a complete understanding of the concepts and techniques.
A float is a slack or cushion or flexibility or margin for delay. It shows time available for delaying an activity without delaying the Project. It would not result in either increase in duration or cost of the project.
For calculating floats, there are different Methods depending upon format of the Network used. There are two types of Net Works: ActivityOnNode (AON) and ActivityonArrow (AOA). Results are same in both cases.
In this hub, only network based on AON would be used.
CONTENT OF A NODE
What is a Node?
Node means box or bud or bulge or knob. In network, such a node is represented by a square normally divided into 9 sectors. These can be increased or decreased as per convenience or requirements. It is not necessary to fill in all the cells or compartment but some can be left empty for any future reference. Each node contains information like Project Description, Project ID, its duration, early start and finish, late start and finish and floats like free float & Total Float. This is shown in the side sheet.
THE QUESTION
To calculate float, an example of Desert Hospital is being given. It has thirteen activities which are shown along their duration in weeks and precedence.
NET WORK DIAGRAM (AoN)
A project network is a graph showing the sequence in which project activities would be completed. It is eventoriented technique not just showing start and finish but the way, the activities are linked together and the manner in which these are to be carried out. It is applied to largescale, onetime, complex, nonroutine projects.
The network technique was first developed by Frederick Taylor and later refined by Henry Ford . DuPont Corporation's CPM was developed at the same time.
From the data given in the question, the following network is drawn. Necessary steps have been discussed in the earlier hub on Float.
Network without floats
Portion of Network
Float Calculation
TYPES OF FLOATS
Floats or slacks are only available in noncritical activities shown linked by blue lines.
To start with two types of floats would be calculated: (i) Free Float (FF) and (ii) Total Float (TF).
FREE FLOAT (FF)
The FF indicates maximum delay from early start of an activity without delaying the earliest start of any of its immediate successor activities. The formula is: FF = ES of immediate succeeding Node (or activity) minus ES start of existing Node minus its duration. It can be shortened to ES of immediate succeeding Node minus EF of the exiting Node. If there are more than one succeeding Nodes, minimum would be the Free Float of the existing Node.
In the side sheet, F has only one succeeding Node, H. Hence ES of H, 7 minus EF of F would be equal to Zero.
E is succeeded by two Nodes i.e. J & K. Therefore, FF would be minimum of the two as shown below:
FF for Node E:
 ES of J – EF of E = 10 – 7 = 3 or
 ES of K – EF of E = 10 – 7 = 3,
 Therefore, FF for E = 3
Likewise FF for Node H would be:
 ES of J – EF of H = 10 – 10 = 0 or
 ES of K – EF of H = 10 – 10 = 0,
 Therefore, FF for H = 0
TOTAL FLOAT (TF)
TF refers to maxim flexibility or margin available for delaying an activity. Here an activity can be started as early as possible and finished as late as possible. Therefore it would provide more flexibility.
Its calculation is easy. For a Node touched by only one tail, it would be difference between EF and LF of the same Node.
TF for E:
 LF of E– EF of E = 12 – 7 = 5
TF for H:
 LF of H – EF of H = 12 – 10 = 2 or
THE NET WORK WITH FLOATS
Constructin of a Mega Project
MORE FLOATS
Three more floats would be discussed now:
 Independent Float
 Interference Float
 Safety Float
INDEPENDENT FLOAT
 Shows the time available even if an activity has a Late Start and Early Finish.
 the delay possible for an activity if all preceding activities start as late as possible whilst all subsequent activities start at their earliest time
 It is most adverse type of float and often results in a negative figure.
In case of Node F, its EF = 7, while its LS=5 and duration is 4. So Independent Float would be 7  5 4 = 2 or Zero which is higher. In other words, if this float is in negative, it is to be shown as zero float or no float.
INTERFERENCE FLOAT
There are two versions of Interference Float.
In the first version, it is like manipulation and it is calculated as Interference Float = TF – Independent Float. While Independent Float is available in all cases, Total Float is available only if an activity is started at its earliest and finished at the latest time. The difference between the two shows the extent to which one can avail margin or flexibility.
Second version is the reduction, total float can bring in the next succeeding activities and is represented by TF – FF.
As per first version, the Inference Float for activities 'F' would be TF  Ind. Float or 20 =0. In the second version, it would remain 2 (TF  FF or 2  0 = 2) as incidentally both Free Float and Independent Float are zero.
SAFETY FLOAT (SF)
The SF of an activity is the leeway for scheduling, all its predecessors without affecting its self. It is calculated as:TF + Int F = FF + SF. One can find out SF, when all other relevant floats are available. (This float is not so popular. It was introduced by Thomas W in his article "Four Floats Measures for critical Path Scheduling."
GLOSSORY
CPM
 Critical Path Method


PERT
 Project Evaluation & Review Technique

AON
 Activity On Node

AOA
 Activity On Arrow

Critical Activities
 Activities having no flexibility or margin for delay

Dummy Activities
 Not a real activity but only a link

Total Float
 Maximum cushion based on early start and late finish

Independent Float
 Minimum float available in any case

Free Float
 Normal float based on early start and early finish

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Comments
thanks a lot
Hi, Thanks for the interesting article.
My question is that there is contradiction in the Independent float definition between this article and your article below:
https://hubpages.com/business/CaseStudyPERTCPM...
where here it states IF=EFLSD
In the other one it's IF=EFLS only (CALCULATIONS OF FLOATS UNDER AON section)
Which one should we use?
Thanks.
Really transparent and vivid explanation............................................................thank you
nice article
thnks it helped me.........:D
i thank you so much sir, i am writing my exams tomorrow,
Thanks sir. You are a great teacher. you create lot of interest to learn with simple and clear method.
I teach Construction Project Management of Pokhara University course. This is helpful for me but i am confused in the relation between independent float and interfering float............
i learn today in class........and now furthr read here. now i can say that i understand very much by this hubs....thank you sir..
now i am able to understand total and free float .thank you very much sir
Dear Sir, this article on Float is also a really comprehensive one. it added to my knowledge a lot.I know about Free Float and Total Float but now i'm able to understand the concept of more kinds of Floats.Thank You so much Sir.
a very well explained article.thnx
There seems a type as well in the free float of Activity D. It should be 0 instead of 4. Thanks.
I think there is an error in ES of Activity E. Should not it be 2 instead of 6? Can you please explain?
I teach at College of the North Atlantic, Canada. For one of my courses namely Project Management and Financial Analysis, I was looking for supplementary material on project float and luckily found this web site. The way this float information is presented here is very interesting and also informative. I thank you for posting such a great deal of material.
Thank you.
sirrrr thankyou so much for thoroughly explaining floats all my concepts are cleared now :) thankyou so much for imparting us with your immense knowledge it will help me a lot for the mid terms JazakAllah
THANX ALOT SIR ....
NO DOUBT ABOUT THAT, SIR U'VE DONE A MIND BLOWING JOB,I'M VERY THANKFUL PLZ SIR SEND ME SUCH MORE INFORMATIVE LINKS LIKE THIS,THAT WILL HELP US MORE IN MAKING ANY KIND OF PROJECT
its nice to see ur work on the hubpages and provid us the latest information regarding the project and new thories of the modrn world.
sir its best way to serve life for education and bettrmnt of the studnt and socity
sir me thnkful to u that u gve me oppertunty to understand the FLOAT through colorful slides & such kind of informative data
Dear Sir,
This hub is explained in much more detail along with sketches, which has cleared all my concepts regarding FLOATS.
Thank you so much sir for providing us that much knowledge.
SANA KARIM
this is very informative all my doubts are cleared sir thanku v much
HIRA SAMI
Dear Sir,
This HUB is a perfect summary of FLOATS that we have learned in your Lectures.
Thanks!
RUFI SHAHZADA
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